This invention relates to marine propulsion devices and more particularly to the tilt and trim systems for such devices.
Marine propulsion devices, such as outboard motors, generally include a hydraulic tilt system for trimming and tilting the propulsion device upwardly about a transverse axis to permit the device to be operated in shallow or vegetation-filled water. Toward this end, the propulsion device is pivotally mounted on a bracket secured to the transom of the boat to be driven. The hydraulic tilt system generally includes a tilt cylinder pivotally connected to the propulsion device and the bracket for pivoting the propulsion device upwardly and one or more relatively short trim cylinders mounted on the bracket and engageable with the motor for tilting the propulsion device through relatively small angles.
The hydraulic trim and tilt cylinders are generally pressurized by a single pump. Initially, both the trim and tilt cylinders are active to trim the propulsion unit through a relatively small angle. After the trim cylinders have reached the end of their stroke, further pivotal movement is under the operation of the tilt cylinder.
When the propulsion device is pivoted upwardly at a relatively high angle, the engine is normally run at idle or a relatively slow speed. However, should the throttle be engaged inadvertently so that the engine begins operating at a high speed, considerable torque will be exerted on the propulsion device, tending to pivot the same downwardly. In order to prevent damage to the hydraulic system, a relief valve is provided which permits the propulsion device to pivot downwardly in the event this thrust-induced pressure exceeds some predetermined value. Under such circumstances, the propulsion device may pivot downwardly at such a rapid rate that the trim cylinders are damaged.